So thats my plan! Its an OOUK 16" f4 with a tube thats not much larger than the mirror. Im convinced stirring the air up will help hugely - my boundary layer is certainly an issue. My impression is this does a better overall job than perhaps 2 fans in holes in the tube wall. Its also less damaging to the structure should I wish to get rid of it!

I've seen examples online but cant seem to find any now so would appreciate any advice or experience, in terms of positioning and suspension methods especially. And recommended cfm.

I'm planning on using something like a 50mm fan (so ~70mm diagonal - theres a 100mm secondary so plenty of shadow to sit in) suspended something like 6-8" above the mirror, with four thin wires, two of which would carry power. Zero chance of aligning perfectly with the spider but I think the extra diffraction is a price worth paying.

Please help!

At f/4, though, off-axis rays hit some of the shadowed primary. So a secondary has an umbra (permanent shadow) and a penumbra (partial shadow).To guarantee the fan doesn't block the light, it will need to be 1/2 or less the diameter of the secondary mirror and exactly centered under the secondary.If you use a centered secondary, after collimation your optical centerline will be tipped toward the focuser slightly, which means the umbral shadow of the secondarywill not be exactly centered on the primary mirror. Likewise, your fan will have to be off-center away from the focuser by a very small amount.

It will be far easier for you to put holes in the tube to both blow air on the primary and evacuate it on the other side.But if you insist on using a fan suspended above the primary, make sure no dimension (including the diagonal if it's square) exceeds 50mm.